Ultrasonograph

ABSTRACT

An ultrasound diagnostic apparatus includes an ultrasound probe, a transmission unit for transmitting an ultrasound signal to an object to be examined via the ultrasound probe, a reception unit for processing a signal received by the ultrasound probe, and an image generating unit for generating an image on the basis of the received signal processed by the reception unit, wherein the transmission unit has a function of transmitting the ultrasound signals with varying frequency plural times in an identical direction at predetermined time intervals. The ultrasound signal transmitted plural times includes a first waveform in which the frequency increases and a second waveform in which the frequency decreases, and the reception unit has a function of phasing and adding received signals respectively corresponding to the first waveform and the second waveform, whereby, in tissue harmonic imaging, the penetration is improved, while the resolution is maintained.

TECHNICAL FIELD

The present invention relates to an ultrasound diagnostic apparatus,and, more particularly, to an ultrasound diagnostic apparatus forimaging a harmonic component that is generated when ultrasound ispropagated inside a body of an object to be examined.

BACKGROUND OF THE INVENTION

An ultrasound diagnostic apparatus is designed to transmit an ultrasoundsignal inside an object to be examined and to obtain informationavailable in diagnosis, e.g. a tomographic image, on the basis of areceived signal, including an echo signal of the transmitted signal.

It has been reported that, in the technique of displaying a tomographicimage, a high-contrast image can be obtained by imaging a harmoniccomponent (e.g. frequency 2 f₀ or 3 f₀) as opposed to a fundamentalcomponent (frequency f₀) of the transmitted signal. An imaging method ofthis type is referred to as “Tissue Harmonic Imaging.”

The above-described harmonic component is generated due to nonlineardistortion occurring mainly when the ultrasound is propagated inside theobject. That is, a signal of the ultrasound irradiated to the inside ofliving body is distorted during propagation in tissue due to a nonlinearresponse of the tissue, and the harmonic component is increased. As aresult, the echo signal includes, e.g. a double frequency 2 f₀ and atriple frequency 3 f₀ of a fundamental component f₀.

In tissue harmonic imaging, it is important how to extract an echo of astrong harmonic component. The conventionally reported methods of tissueharmonic imaging include one referred to as a “filtering technique”, forexample.

This technique is designed to extract the harmonic component of e.g. 2f₀ from a received signal using a band pass filter with a centralfrequency of e.g. 2 f₀. Another example is a method referred to as a“pulse inversion technique,” which is designed to emphasize the secondharmonic component by transmitting first and second waveforms having amutually alternated polarity at predetermined time intervals, andphasing and adding the echo signals thereof to cancel the fundamentalcomponent. Further, for example, Japanese Unexamined Patent PublicationNo. 2002-34976 discloses a technique of extracting a harmonic componentthrough a filter from received signals obtained from transmitted signalshaving two different center frequencies, and widening the band of theharmonic component by combining those extracted signals, thus improvingthe resolution and enhancing the signal strength in a beam depthdirection, as well as suppressing generation of motion artifacts.However, because the frequency of the harmonic component in anultrasound signal is higher than that of the fundamental component, theharmonic component is sensitive to attenuation of the signal duringpropagation. Accordingly, the degree of an echo signal received from adeep portion, i.e. the penetration, is undesirable. Meanwhile, when thecenter frequency f₀ of the fundamental component is lowered, the echosignal is scarcely affected by the attenuation, whereby the penetrationcan be improved. However, the resolution is deteriorated, as isgenerally known.

SUMMARY OF THE INVENTION

The present invention has been devised in consideration of theabove-described factors, and an object thereof is to improve thepenetration, while maintaining the resolution.

To achieve the above-stated object, an ultrasound diagnostic apparatusaccording to the present invention includes an ultrasound probe, atransmission unit for transmitting an ultrasound signal to an object tobe examined via the ultrasound probe, a reception unit for processing asignal received by the ultrasound probe, and an image generating unitfor generating an image on the basis of the received signal processed bythe reception unit, wherein the transmission unit has a function oftransmitting the ultrasound signal having a varying frequency pluraltimes in one direction at predetermined time intervals. This ultrasoundsignal transmitted plural times includes a first waveform in which thefrequency increases and a second waveform in which the frequencydecreases, and the reception unit has a function of phasing and addingreceived signals corresponding to the first waveform and the secondwaveform.

Accordingly, in comparison with a conventional case where a waveformhaving a frequency f₀ is transmitted while its polarity is alternated, afrequency spectrum of the received signal after phasing and additionchanges, whereby a component in a frequency band between f₀ and 2 f₀ canbe emphasized. Because the component in this frequency band has afrequency lower than 2 f₀, it is scarcely affected by attenuation, andso the penetration is desirable. Accordingly, by extracting thisfrequency component to generate an image based thereon, the penetrationin the tissue harmonic imaging can be improved without lowering f₀,i.e., without lowering the resolution.

Here, the waveform having a varying frequency may be formed by, e.g.,joining each one cycle or a plurality of cycles of waveforms havingdifferent frequencies. Alternatively, it may be formed by joining partsof ½, ¼, or ⅛ cycle of the waveforms having different frequencies, or achirp waveform in which the frequency sequentially changes may be used.

In this case, the above-described frequency spectrum is varied byvariably setting a change rate of frequency variation of the firstwaveform and the second waveform. Here, particularly when diagnosticinformation is acquired from a deep portion inside the object (portiondistant from the probe), the frequency spectrum of a received signal isdesirably shifted so as to be low in consideration of the penetration.Then, it is desirable that the transmission unit has a function ofvariably setting a transmission focus depth, and the change rate offrequency variation of the first waveform and the second waveform isvariably set depending on the transmission focus depth.

Further, in this case, when the first waveform and the second waveformrespectively shift so that their signal strengths decrease, theabove-described spectrum variation can be emphasized. At this time, itis also desirable that the change rate of the signal strengths of thefirst waveform and the second waveform is variably set depending on thetransmission focus depth.

Further, it is also desirable that the polarities of the first waveformand the second waveform are mutually alternated, and the reception unithas a function of phasing and adding the reception signals correspondingto the first waveform and the second waveform after amplifying orattenuating them with a gain difference.

Even when the above-described technique is utilized, the frequencyspectrum of the received signal, after phasing and addition, changes,and the component in the frequency band between f₀ and 2 f₀ can beemphasized, whereby the penetration can be improved without lowering f₀,i.e., without lowering the resolution. In this case, the gain differencemay be variably set depending on the reception timing, i.e., the areception focus depth. For instance, it is desirable that the receptionsignals corresponding to the first waveform and the second waveform aresubjected to a time gain control with different correlation curvesbetween reception timing and gain.

Further, the reception unit may have a function of performing areception focus processing on the received signal and include a filterfor extracting a predetermined frequency band of the received signal, inwhich the frequency band is variably set depending on the predeterminedreception focus depth. According thereto, it is possible to adapt to thespectrum variation of the received signal occurring due to thedifference in the reception focus depth, i.e. the difference in depth ofthe portion examined inside the object, and to extract a component in apreferable frequency band.

Further, in the case where the change rate of the varying frequency orsignal intensity of the first waveform and the second waveform accordingto the predetermined transmission focus depth is variably set, afrequency band of the filter may be variably set in accordance with thetransmission focus depth in order to adapt to the spectrum variation ofa received signal caused by varying the frequencies or the signalintensities, as mentioned above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the structure of an ultrasounddiagnostic apparatus according to a first embodiment of the presentinvention.

FIGS. 2 a and 2 b are graphs presenting the simulation results ofwaveforms of a transmitted signal and spectrums of the transmittedsignal and a received signal based on a conventional pulse inversionmethod.

FIGS. 3 a and 3 b are presenting the simulation results of waveforms ofa transmitted signal and spectrums of the transmitted signal and areceived signal in the ultrasound diagnostic apparatus of FIG. 1.

FIGS. 4 a and 4 b are graphs showing variation of a reception spectrumoccurring due to a depth of received signal generation.

FIGS. 5 a and 5 b are graphs presenting the simulation results ofwaveforms of a transmitted signal and spectrums of the transmittedsignal and a received signal in an ultrasound diagnostic apparatusaccording to a second embodiment of the present invention.

FIG. 6 is a block diagram showing the structure of an ultrasounddiagnostic apparatus according to a third embodiment of the presentinvention.

FIG. 7 is a graph presenting the simulation results of spectrums of atransmitted signal and a received signal in the ultrasound diagnosticapparatus according to the third embodiment of the present invention.

FIG. 8 is a graph presenting the simulation results of spectrums of atransmitted signal and a received signal in the ultrasound diagnosticapparatus according to the third embodiment of the present invention.

FIG. 9 is a graph showing the simulation results of spectrums of atransmitted signal and a received signal in the ultrasound diagnosticapparatus according to the third embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE EMBODIMENT Embodiment 1

Hereinafter, an ultrasound diagnostic apparatus according to a firstembodiment of the present invention will be described. FIG. 1 is a blockdiagram showing the structure of the ultrasound diagnostic apparatusaccording to this embodiment. As shown in FIG. 1, the ultrasounddiagnostic apparatus includes an ultrasound probe 1, a transmission unit3 for transmitting an ultrasound signal to an object to be examined (notshown in the figure) via the ultrasound probe 1, a reception unit 5 forreceiving a received signal including an echo signal from the object viathe ultrasound probe 1 and for processing the received signal, and animage producing/display unit 7 for generating and displaying adiagnostic image on the basis of the received signal processed by thereception unit 5. The image producing/display unit 7 includes a videoprocessing unit for performing detection, compression, and the like, aDoppler processing unit, and a scan converter, which are not shown inthe figure.

The transmission unit 3 includes an arbitrary waveform generator 9having a function of generating a transmitted signal combining aplurality of waveforms respectively having a predetermined amplitude,frequency, and starting phase, a time axis controller 11 having afunction of time-inverting the waveform output by the arbitrary waveformgenerator 9, and a transmitter 13 having a power amplifier, forsupplying a driving signal to the ultrasound probe 1 in response to anoutput signal of the time axis controller 11. Meanwhile, the time axiscontroller 11 has a so-called first-in/first-out function and afirst-in/last-out function using the output of the arbitrary waveformgenerator 9 as an input signal, and it has a shift register for thesefunctions.

Reception unit 5 receives a signal output from ultrasound probe 1, andit includes a receiver 15 having a pre-amplifier, a time gain control(TGC) amplifier, and an A/D converter, which are not shown in thefigure. The reception unit 5 further includes a phasing/addition unit 17for phasing and adding received signals of channels output by receiver15 corresponding to the respective transducers of the ultrasound probe 1and for outputting the added signals as an RF line signal; a line adder19 for keeping the received signal previously output fromphasing/addition unit 17, RF-adding it and a time-delayed receivedsignal output afterward from phasing/adding unit 17, in consideration oftheir phase, and outputting the resultant signal; and a band pass filter21 having a function of performing a band pass digital filtercalculation for extracting a particular frequency band signal from amongthe output signals of the line adder 19. A so-called digital beam formeris used as the phasing/addition unit 17 to minimize distortion occurringin the addition processing. Further, the system controller 23 isprovided for totalizing and controlling operations of each component inthe transmission unit 3, the reception unit 5, and the imageproducing/display unit 7 described above. Further, ultrasound probe 1includes a plurality of transducers 25 arranged to face the object (notshown) in a line or in a plane.

Next, the operation of the above-described ultrasound diagnosticapparatus will be described. First, arbitrary waveform generator 9generates and outputs a waveform of a transmitted signal on the basis ofa command received from the system controller 23. The waveform of thesignal output by the arbitrary waveform generator 9 is formed bycombining waveforms in which the frequency is time-sequentially varied.This point will be described later in more detail. A signal output bythe arbitrary generator 9 is input to the time axis controller 11. And,the first waveform is output without being time-inverted by thefirst-in/first-out function. After that, the time axis controller 11outputs a second waveform formed by inverting the first waveformsymmetrically about a line perpendicular to a time axis by thefirst-in/last-out function with a time delay relative to the firstwaveform. Transmitter 13 performs a known transmission focus processingon the basis of the first waveform and the second waveform, and itgenerates and outputs a driving signal to each transducer 25 of theultrasound probe 1. A transducer 25 that is supplied with the drivingsignal from transmitter 13 via a transmission/reception separatingcircuit (not shown) is respectively oscillated to generate an ultrasoundwave, and ultrasound beams proceeding in a direction in which the wavesurfaces of the respective ultrasound waves transmitted by transducers25 coincide with each other are formed inside the object (not shown).

On the other hand, the ultrasound signal propagating inside the objectas an ultrasound beam is reflected at a portion having a differentacoustic impedance inside the object. The reflected signals return tothe ultrasound probe 1 and are received as a received signal. Thereceived signals are converted from sound waves into electric signals bythe transducer 25 and are input to receiver 15 via thetransmission/reception separating circuit (not shown). In receiver 15,the received signal of a channel corresponding to each transducer 25 isamplified by the pre-amplifier and the TGC amplifier, is A/D converted,and then output. A signal output from receiver 15 is input tophasing/addition unit 17, subjected to a known dynamic focus processingfor sequentially correcting any variation in reception timing of thereceived signal between the respective channels occurring due to adifference in the distance from a generating portion of the receivedsignal to each transducer 25, and the processed signals are added andoutput. Those processings concerning the ultrasound reception areperformed on each of the received signals corresponding to the firstwaveform and the second waveform. The received signals corresponding tothe first waveform and the second waveform are combined by line adder19, temporarily keeping and time-delaying the received signalcorresponding to the first waveform and adding it and the receivedsignal corresponding to the second waveform, and are output as acombined received signal. In band pass filter 21, a component of thepredetermined frequency band of the combined received signal isextracted. Then, the image producing/display unit 7 generates anddisplays an ultrasound diagnostic image on the basis of a signal of theextracted frequency band component. That is, the ultrasound diagnosticapparatus carries out the above-described operations while performingscans in the beam direction, and the image producing/display unit 7performs video processings, including detection and compression, Dopplersignal processing, scan conversion and the like, to produce a knownB-mode or Doppler-mode image. Meanwhile, the system controller 23 notonly controls the above-described series of operations, but alsogenerates data for arbitrary waveform generator 9.

Next, the first waveform, the second waveform, and the frequencyspectrums of a transmitted signal and received signal according to thisembodiment will be described. First, for easy understanding of thecharacteristics of this embodiment, the simulation results of the firstwaveform, the second waveform, and the frequency spectrums of atransmitted signal and the received signal in the conventional pulseinversion method, which are shown in FIGS. 2 a and 2 b, will bedescribed. FIG. 2 a is a graph presenting the first waveform and thesecond waveform, in which the horizontal axis indicates time and thevertical axis indicates sound pressure of the transmitted wave. Here,the first waveform is presented by a full line and the second waveformis presented by a broken line. As shown in FIG. 2 a, both the firstwaveform and the second waveform are sequences of sine waves for twocycles having the same frequency f₀ (2 MHz), to both of which a Hanningweight is applied to simulate a waveform in a living body. The polarityof the first waveform is opposite to that of the second waveform. Thatis, the sound pressure decreases at the start of a signal with thepolarity of the first waveform, and with the polarity of the secondwaveform, the sound pressure increases.

FIG. 2 b is a graph presenting the frequency spectrum of the transmittedsignal and the received signal obtained by phasing and adding thereceived signals corresponding to the first waveform and the secondwaveform. In the graph, the horizontal axis indicates a frequency ratio(f/f₀) in relation to f₀=2.0 MHz and the vertical axis indicates signalstrength (dB). In FIG. 2 b, the spectrum of the transmitted signal ispresented by a broken line and that of a combined reception signal ispresented by a full line.

As shown in FIG. 2 b, in the spectrum of the transmitted signal, thesignal strength increases when the frequency increases from 0, and ithas its first peak at frequency f₀, where it becomes maximal. And, whenthe frequency further increases, the signal strength then decreases andbecomes −40 dB of the first peak at frequency 2 f₀. When the frequencyfurther increases from frequency 2 f₀, the signal strength againincreases and has its second peak in the vicinity of frequency 2.3 f₀,and again decreases after that. The signal strength of the second peakis approximately −32 dB of the first peak.

Meanwhile, the spectrum of the combined reception signal has its peakswhere the signal strength becomes maximal in the vicinity of 0.6 f₀, 2f₀, and 4 f₀, and has its nadirs where the signal strength becomesminimal in the vicinity of 1.2 f₀ and 3.3 f₀. Among the peaks, thesignal strength is maximized at the peak in the vicinity of 2 f₀, andboth signal strengths at the peaks in the vicinity of 0.6 f₀ and 4 f₀are −14 dB of the signal strength at the peak in the vicinity of 2 f₀.On the other hand, the signal strengths at the nadirs in the vicinity of1.2 f₀ and 3.3 f₀, where the signal strength becomes minimal, arerespectively about −28 dB and −21 dB of that at the peak at 2 f₀.

Next, one example of the first waveform and the second waveform havingvarying frequencies and the frequency spectrums of a transmitted signaland a received signal in the ultrasound diagnostic apparatus accordingto this embodiment of the present invention will be described withreference to the simulation results shown in FIGS. 3 a and 3 b. FIG. 3 ais a graph showing the first waveform and the second waveform, where thehorizontal axis indicates time and the vertical axis indicates soundpressure. Here, the first waveform is presented by a solid line and thesecond waveform is presented by a broken line. As shown in FIG. 3 a, thefirst waveform is formed by joining a first cycle of frequency f₁(=1.8MHz) and a second cycle of frequency f₂(=2.2 MHz) with a polarity inwhich the sound pressure increases at the start of the signal. On theother hand, the second waveform is formed by joining a first cycle offrequency f₂ and a second cycle of frequency f₁, in which the waveformshifts at a certain change rate between those cycles, and with apolarity in which the sound pressure decreases at the start of signal.Furthermore, a Hanning weight similar to the waveform shown in FIG. 2 ais applied to both those first and second waveforms. In other words, thesecond waveform is formed by time-inverting the first waveform.

FIG. 3 b is a graph showing the frequency spectrums of the transmittedsignal and a received signal obtained by phasing and adding receivedsignals corresponding to the first waveform and the second waveform,wherein the horizontal axis indicates the frequency ratio (f/f₀) inrelation to f₀=2.0 MHz and the vertical axis indicates signal strength(dB), as in FIG. 2 b. In the graph of FIG. 3 b, the spectrum of thetransmitted signal is presented by a broken line and that of thereceived signal obtained by phasing and addition is presented by a solidline.

As shown in FIG. 3 b, in the spectrum of the transmitted signal, thesignal strength increases when the frequency increases from 0 and hasits peak where it becomes maximal at frequency f₀. When the frequencyfurther increases, the signal strength decreases. After the decreasingrate of signal strength becomes small in the vicinity of frequency 2 f₀and then becomes 0, the signal strength again decreases, while thedecreasing rate increases. Meanwhile, the signal strength in thevicinity of the frequency 2 f₀ is approximately −23 dB relative to thepeak at frequency f₀.

On the other hand, in the spectrum of the phased and added receivedsignal, the signal strength has its peaks where it becomes maximal inthe vicinity of 0.4 f, 1.6 f₀, and 2.8 f₀, and has its nadirs where itbecomes minimal in the vicinity of 0.7 f₀, 2.2 f₀, and 3.7 f₀. Among thepeaks where the signal strength becomes maximal, the signal strength ismaximized at the peak in the vicinity of 1.6 f₀. The signal strengths atthe peaks in the vicinity of 0.4 f₀ and 2.8 f₀ are respectively about−12 dB and −2 dB of the signal strength at the peak at 1.6 f₀.Meanwhile, among the nadirs where the signal strength becomes minimal,the signal strength is approximately −14 dB in the vicinity of 0.7 f₀,−17 dB in the vicinity of 2.2 f₀, and −35 dB in the vicinity of 3.7 f₀.

As is clear from a comparison of FIG. 3 b with FIG. 2 b, in theconventional pulse inversion method, a frequency component in thevicinity of 2 f₀ is most emphasized when the received signalscorresponding to the first waveform and the second waveform arecombined. Meanwhile, the peak of signal strength comes into the vicinityof 1.6 f₀ by differently determining f₁ and f₂, and the frequencyspectrum thus shifts so as to be low.

On the other hand, in the ultrasound diagnostic apparatus according tothis embodiment, the difference Δf(=|f₁−f₂|) between frequencies f₁ andf₂ is variably set depending on the predetermined transmission focusdepth. Specifically, Δf is variably set so that the frequency spectrumof the signal combining the received signals corresponding to the firstwaveform and the second waveform shifts so as to be low as thetransmission focus depth becomes deep. Meanwhile, the average frequencyof the frequencies f₁ and f₂ is fixed to be f₀. When the transmissionfocus depth is shallow, the transmission and reception are performedwhile Δf is set to be 0, as in the conventional pulse inversion method.Δf is varied as the transmission focus depth becomes deep and atransmitted wave is generated so that the above-mentioned peak of thefrequency spectrum shifts from 2 f₀ to e.g. f₀, desirably from 2 f₀ to,e.g. 1.5 f₀. For example, in the case of performing a multipletransmission focus to complete a received signal for one beam line bycombining a plurality of ultrasound beams having different transmissionfocus depths, Δf is variably set depending on the focus depth of eachbeam. For example, when the focusing is performed on three points, Δf oneach point is set so that the peaks of its frequency spectrum are ate.g. 2 f₀, 1.8 f₀, and 1.6 f₀ in the ascending order of focus depth.This setting of Δf, depending on the transmission focus depth, can becarried out by simulation or experiment using an ultrasonic phantom.

Further, in the ultrasound diagnostic apparatus according to thisembodiment, a frequency pass band of band pass filter 21 is variably setdepending on the variation of the above-described frequency spectrum.Specifically, in order to adapt to the spectrum shift of a receivedsignal after phasing and addition, the frequency pass band is shifted soas to be low as the transmission focus depth becomes deep.

Further, the frequency pass band of the band pass filter 21 is variablyset depending on the reception focus depth, even when the transmittedsignal is not changed. FIGS. 4 a and 4 b are graphs showing a receptionspectrum of a signal received from a shallow portion of the object, i.e.a portion close to the ultrasound probe, and a reception spectrum of asignal received from a deep portion of the object, i.e. a portiondistant from the ultrasound probe. In FIGS. 4 a and 4 b, only 2 f₀ ispresented as a harmonic component for easy understanding. Since thetransmitted signal is a pulse wave having usually a few cycles, thespectrum has a certain degree of band around f₀ and 2 f₀, as seen inFIGS. 4 a and 4 b. As shown in FIG. 4 b, in the spectrum of a receivedsignal from a deep portion, the harmonic component is increased by anonlinear distortion occurring when the ultrasound propagates in theliving body. However, in the spectrum distribution of the harmoniccomponent, the frequency spectrum shifts so as to be low and the centerfrequency lowers because a higher frequency component is more sensitiveto the attenuation in propagation. Therefore, to deal with this, thefrequency pass band of the band pass filter 21 is set to shift to be lowas the reception focus depth becomes deeper in conjunction with thedynamic focusing in reception. Specifically, in the case of using adigital FIR filter as the band pass filter 21, its coefficient isdesirably set variably in conjunction with the reception focus depth.

As described above, according to this embodiment, by phasing and addingthe received signals respectively corresponding to the first waveform inwhich the frequency shifts so as to increase and the second waveform inwhich the frequency shifts so as to decrease, the frequency component inthe band between f₀ and 2 f₀ is emphasized. Accordingly, the waveform isless affected by the attenuation than in the conventional pulseinversion method of emphasizing 2 f₀, whereby the penetration can beimproved in a deep portion of focus. Besides, since the 2 f₀ componentis imaged, the resolution can be maintained in a shallow portion.

Further, since the change rate of frequency variation of the firstwaveform and the second waveform is variably set depending on thetransmission focus depth, it is possible to emphasize a relativelyhigh-frequency component at a shallow focus depth in consideration ofcontrast and to emphasize a relatively low frequency component at a deepfocus depth in consideration of penetration, whereby a desirable imagecan be produced depending on the predetermined transmission focus depth.

Further, a frequency band pass filter calculation is performed on thereceived signal after phasing and addition to variably set the frequencypass band depending on the transmission focus depth, whereby thefrequency band to be emphasized can be extracted in response to thechange rate of frequency variation of the transmitted wave.

Further, since the frequency pass band is variably set also depending onthe reception focus depth, a frequency band can be extracted in responseto the spectrum variation of a received signal occurring due to adifference in attenuation of the signal caused by a difference in thepropagation distance.

Further, for example, when a relatively shallow portion where thepenetration is not a critical problem is examined, an image may beproduced by extracting a component of frequency higher than 2 f₀. Forexample, although in FIG. 3 b there is a peak also in the vicinity of2.8 f₀, the signal strength at the peak where the frequency is largerthan 2 f₀ may be extracted using a band pass filter and used in imagegeneration. According thereto, a desirable image contrast can beobtained when a shallow portion is examined.

Further, when a shallow portion is examined, the center frequency of thetransmitted signal may be set to be high. In the example of FIGS. 3 aand 3 b, the center frequency is set as f₀=2.0 MHz and the firstwaveform and the second waveform are f₁=1.8 MHz and f₂=2.2 MHz,respectively. However, they may be set such that the center frequencyf₀=2.1 MHz, f₁=2.0 MHz, and f₂=2.2 MHz. According thereto, ahigh-contrast image can be obtained at a relatively shallow depth wherethe penetration is not a problem.

Meanwhile, according to the above-described embodiment, the secondwaveform is obtained by time-inverting the first waveform using the timeaxis controller. However, when the arbitrary waveform generator candirectly generate the second waveform, the time axis controller isunnecessary.

Further, according to the above-described embodiment, the first waveformand the second waveform are respectively formed by joining waveforms fortwo cycles. However, they may be formed by joining waveforms for threeor more cycles. For example, the first waveform may be formed by joiningeach one cycle of the waveforms of f₁=1.8 MHz, f₂=2.0 MHz, and f₃=2.2MHz, and the second waveform may be obtained by time-inverting the firstwaveform. In this manner, when the first waveform is a combination ofeach one cycle of the center frequency f₁, f₂ . . . f_(n), . . . f_(N)(N≧2), where f₁<f₂< . . . <f_(n)< . . . <f_(N), and the second waveformis an inversion of the first waveform relative to the time axis, theeffect of the invention is not lost even if N=4. However, because thedifference between those two transmission waves becomes relatively smallwhen the wave number increases, it can be that the invention isespecially effective when N<6.

Further, according to the above-described embodiment, the first waveformand the second waveform are formed by combining sine waves in which thefrequency is varied at every one cycle. However, the frequency may bevaried at every two or more cycles. Alternatively, it may be frequentlychanged, such as at every ½ cycle or ¼ cycle, or it may be a so-calledchirp waveform in which the frequency sequentially changes.

Embodiment 2

Next, a second embodiment of the ultrasound diagnostic apparatusaccording to the present invention will be described. The same points asthose enumerated in the description of the first embodiment will not bementioned, again, and only the differences will be described. Theultrasound diagnostic apparatus according to this embodiment ischaracterized in that the amplitude of both the first waveform and thesecond waveform is changed. That is, this embodiment is characterized inthat the amplitude of waveform in the first cycle of the first waveformand the second waveform is respectively preset to be larger than that ofthe subsequent waveform.

An example of the first waveform and the second waveform having avarying frequency and amplitude and the frequency spectrum oftransmitted signal and received signal will be described with referenceto FIGS. 5 a and 5 b showing the simulation results thereof. FIG. 5 a isa graph presenting the first waveform and the second waveform, in whichthe horizontal axis indicates time and the vertical axis indicates soundpressure. In the graph, the first waveform is presented by a solid lineand the second waveform is presented by a broken line. As shown in FIG.5 a, the first waveform is formed by joining a first cycle of frequencyf₁(=1.8 MHz) and a second cycle of frequency f₂(=2.2 MHz), and, with itspolarity, the sound pressure decreases at the start of the signal. Onthe other hand, the second waveform is formed by joining a first cycleof frequency f₂ and a second cycle of frequency f₁, and, with itspolarity, the sound pressure increases. In both the first waveform andthe second waveform, the amplitude A2 in the second cycle is set to besmaller than the amplitude A1 in the first cycle. For example, in thecase of FIG. 5 a, the amplitude is set as A2=0.9A1.

FIG. 5 b is a graph presenting frequency spectrums of a transmittedsignal and a received signal obtained by phasing and adding receivedsignals respectively corresponding to the first waveform and the secondwaveform. Similar to FIG. 2 b, the horizontal axis indicates thefrequency ratio (f/f₀) in relation to f₀=2.0 MHz and the vertical axisindicates signal strength (dB). In FIG. 5(b), the spectrum of thetransmitted signal is presented by a broken line, and the spectrum of aphased and added received signal is presented by a solid line.

As shown in FIG. 5 b, the spectrum of transmitted wave resembles thatshown in FIG. 3 b. However, the signal strength in the vicinity offrequency 2 f₀ is −25 dB of the signal strength in the vicinity of f₀.

On the other hand, the spectrum of the phased and added received signalhas its peaks where it becomes maximal in the vicinity of 0.4 f₀, 1.6f₀, and 2.8 f₀, and has its nadirs where it becomes minimal in thevicinity of 0.7 f₀, 2.2 f₀, and 3.8 f₀. Among the maximal peaks, thesignal strength is maximized at the peak in the vicinity of 1.6 f₀.Compared therewith, the signal strengths at the peaks in the vicinity of0.4 f₀ and 2.8 f₀ are respectively about −13 dB and −2 dB. Meanwhile,the signal strengths at the nadirs in the vicinity of 0.7 f₀, 2.2 f₀,and 3.8 f₀ where the signal strength becomes minimal are respectivelyabout −17 dB, −10 dB, and −40 dB or less.

As is clear by comparing FIG. 5 b with FIG. 2 b, the peak of the signalstrength comes in the vicinity of 1.6 f₀ by shifting the frequency f₁and frequency f₂ and shifting the amplitude A1 and amplitude A2, and thefrequency spectrum shifts so as to be low.

Further, in the frequency spectrum according to this embodiment,difference Δf between the frequency f₁ and the frequency f₂ is variablyset in response to a predetermined transmission focus depth, and theratio A2/A1 between the amplitude A1 and the amplitude A2 is alsovariably set. Specifically, as in the first embodiment, Δf and A2/A1 arevariably set so that the frequency spectrum of a signal obtained bycombining received signals corresponding to the first waveform and thesecond waveform shifts so as to be low as the transmission focus depthbecomes deeper.

As described above, according to this embodiment, the spectrum shift ofthe phased and added received signal can be emphasized by shifting theamplitude of the first waveform and the second waveform, in addition tothe effect obtained by first embodiment described above.

Embodiment 3

Next, a third embodiment of the ultrasound diagnostic apparatusaccording to the present invention will be described. Here again, thesame points as those enumerated in the description of the firstembodiment will not be mentioned again, and only the differences will bedescribed.

FIG. 6 is a block diagram showing the structure of an ultrasounddiagnostic apparatus according to this embodiment. As shown in FIG. 6,the ultrasound diagnostic apparatus includes an ultrasound probe 31having a plurality of ultrasound transducers or an ultrasound transducerarray (not shown), a pulse inversion control unit 33 for controlling asignal transmitted to an object to be examined (not shown) via theultrasound probe 31, and a transmitted wave phasing circuit 35 forgenerating a transmitted wave in response to a command from the pulseinversion control unit 33 and the driving ultrasound probe 31. Thetransmission phasing circuit 35 has a transmission timing generatingcircuit, a transmission beam former circuit, and a transmission driver,which are not shown, and it is designed to supply a high-pressuretransmitted signal to the ultrasound probe 31. At this time, thetransmitted beam former circuit generates a beam forming signal forforming an ultrasound beam in a predetermined direction on the basis ofthe transmission timing signal generated by the transmission timinggenerating circuit. The beam forming signal includes a plurality ofdriving signals, each of which is given a time difference correspondingto the predetermined direction of the beam.

Further, the apparatus also includes a reception phasing circuit 37 forphasing and adding the signals received from the object via theultrasound probe at each channel of the plurality of ultrasoundtransducers, a variable gain circuit 39 for amplifying or attenuating asignal output by the reception phasing circuit 37 depending on thevariably determined gain, and a two beam adder unit 41 for temporarilystoring an output signal of the variable gain circuit 39 and phasing andadding it with a signal output by the variable gain circuit 39 after atime interval. Further, the apparatus includes a variable band passfilter 43 for performing a digital band pass filter calculation on theoutput signal of two beam adder unit 41, a B-mode processing unit 45 forperforming known B-mode image processings including detection,logarithmic compression, and enhancement processing on the basis of theoutput signal of the variable band pass filter 43, a DSC circuit 47, anda monitor 49 for displaying an image of video signals output by the DSCcircuit 47. Further, the apparatus includes a control unit 51 forcontrolling the variable gain circuit 39 and variable band pass filter43, and it is connected to a console 53 having an input means.

Meanwhile, the variable gain circuit 39 has a function of performing aknown time gain control with variable gain on the plurality of signalsreceived at time intervals in accordance with instructions from thepulse inversion control unit 33 and the control unit 51. Further, thevariable band pass filter 43 has a reception dynamic filter function forvariably setting the center frequency and the band width of thefrequency pass band depending on the reception depth on the basis of atime control signal generated by the control unit 51.

Next, the operation of the above described ultrasound diagnosticapparatus will be described. In the ultrasound diagnostic apparatusaccording to this embodiment, the first waveform and second waveform aresimilar to those shown in FIG. 2 a, i.e. the first waveform is formed byjoining two waveforms of frequency f₀ and the second waveform is formedby time-inverting or polarity-inverting the first waveform, as in theconventional pulse inversion method. In the variable gain circuit 39, atime gain control is performed on received signals respectivelycorresponding to the first waveform and the second waveform in areception timing of the received signals, i.e. with a gain (amplitudegain) different depending on the depth of the detecting portion. Afterthat, those received signals are phased and added by the two beam adderunit 41 so as to be formed into one RF signal.

FIGS. 7 to 9 are graphs showing the simulation results of the frequencyspectrums of the transmitted signal and the combined received signalaccording to this embodiment, wherein the gain ratio between thereceived signals corresponding to the first waveform and the secondwaveform is 1:1, 1.2:0.8, and 1.35:0.65, respectively. In each figure,as in FIG. 2 b, the horizontal axis indicates the frequency ratio (f/f₀)in relation to f₀=2 MHz and the vertical axis indicates signal strength(dB). The spectrum of the transmitted signal is represented by a brokenline and that of received signal, after phasing and addition, isrepresented by a solid line. Meanwhile, needless to say, the spectrum ofthe transmitted signal here is the same as that shown in FIG. 2 b.

Next, the spectrum of the received signal after phasing and addition ineach figure will be described. FIG. 7 shows the case where a ratio ofthe gain (hereinafter referred to as “gain ratio”) between receivedsignals respectively corresponding to the first waveform and the secondwaveform is 1:1. As shown in FIG. 7, the spectrum of the received signalhas its peaks where the signal strength becomes maximal in the vicinityof 0.6 f₀, 1.9 f₀, and 3.7 f₀. On the other hand, the spectrum has itsnadirs where the signal strength becomes minimal in the vicinity of 1.1f₀ and 2.9 f₀. The signal strength is maximized at the peak in thevicinity of 1.9 f₀. In relation to this maximum signal strength, thesignal strengths at other peaks and at the nadirs are about −9 dB, −25dB, −17 dB, and −6 dB, respectively, in the vicinity of 0.6 f₀, 1.1 f₀,2.9 f₀, and 3.7 f₀.

FIG. 8 shows the case where the gain ratio is 1.2:0.8. As shown in FIG.8, the spectrum of the received signal has its peaks where the signalstrength becomes maximal in the vicinity of 0.6 f₀, 1.9 f₀, and 3.7 f₀.On the other hand, the spectrum has its nadirs where the signal strengthbecomes minimal in the vicinity of f₀ and 2.9 f₀. The signal strength ismaximized at the peak in the vicinity of 1.9 f₀. In relation to themaximum signal strength, the signal strengths at other peaks and atnadirs are about −8 dB, −15 dB, −13 dB, and −5 dB, respectively, in thevicinity of 0.6 f₀, f₀, 2.9 f₀, and 3.7 f₀.

FIG. 9 shows the case where the gain ratio is 0.35:0.65. As shown inFIG. 9, the spectrum of the received signal has its peak where thesignal strength becomes maximal in the vicinity of 0.6 f₀, 1.9 f₀, and3.7 f₀. On the other hand, the spectrum has its nadirs where the signalstrength becomes minimal in the vicinity of 0.9 f₀ and 2.9 f₀. Thesignal strength is maximized at the peak in the vicinity of 1.9 f₀. Inrelation to the signal strength at this time, the signal strengths atother peaks and at nadirs are −8 dB, −9 dB, −10 dB, and −5 dB,respectively, in the vicinity of 0.6 f₀, 0.9 f₀, 2.9 f₀, and 3.7 f₀.

As is clear by comparing FIG. 7, FIG. 8 and FIG. 9, the spectrum of thephased and added received signal is varied by changing the gain ratio.For example, when attention is focused on the component in a frequencyband between f₀ and 2 f₀, it is more emphasized in FIG. 8 than in FIG.7, and it is further emphasized in FIG. 9 than in FIG. 8. Meanwhile, inrelation to the peak at 1.9 f₀, the signal strength at 1.5 f₀ is about−7 dB in FIG. 7, −5 dB in FIG. 8, and −3 dB in FIG. 9.

Further, according to this embodiment, the gain ratio is variably set inaccordance with a predetermined reception focus depth. Specifically, thegain ratio is set to be larger as the reception focus depth becomesdeeper. This change of gain ratio is sequentially carried out in thereception timing in conjunction with a known reception dynamic focus.That is, the time gain control is performed with different correlationcurves between the reception timing and the gain on the received signalsrespectively corresponding to the first waveform and the secondwaveform.

Further, variable band pass filter 43 variably sets a frequency passband depending on a predetermined reception focus depth. Specifically,the frequency pass band is set to be wide and the center frequency isset to be low in an area where the reception focus depth is shallow,and, thus, a secondary harmonic component is passed through a wide band.As the reception focus depth becomes deeper and the fundamentalcomponent is more emphasized, the frequency pass band of the variableband pass filter is set to be narrow and a floor frequency is set to behigh, thereby the fundamental component is reduced.

As described above, according to this embodiment, the polarities of thefirst waveform and the second waveform are mutually inverted, and thereceived signals corresponding to the first waveform and of the secondwaveform are amplified or attenuated with a gain difference. In thismanner, the frequency spectrum of the received signal after phasing andaddition is varied, and a component in the frequency band between f₀ and2 f₀ can be emphasized, whereby the penetration can be improved withoutlowering F₀ and deteriorating the resolution.

Further, since the gain difference and the frequency pass band of thevariable band pass filter are variably set depending on the receptionfocus depth, in a relatively shallow portion the secondary harmoniccomponent can be emphasized by canceling the fundamental component bysetting the gain difference to be small or 0, or by setting thefrequency pass band of the variable band pass filter to be wide. On thecontrary, in a relatively deep portion, the penetration can be improvedby setting the gain difference to be large, narrowing the frequency passband, and setting the center frequency to be low.

Meanwhile, according to the above-described embodiment, the gains ofboth received signals corresponding to the first waveform and the secondwaveform are variably set. However, it is also possible that one gain isfixed and only the other is variably set. For example, it is possiblethat the gain of the first waveform is fixed as 1 and only the gain ofthe second waveform is variably set. Alternatively, the gain ratio maybe variably set in stages as 1:1, 1:0.6, and then 1:0.3. Further, inaddition to or instead of varying the gain of a received signal, theamplitude in transmission of the first waveform and the second waveformmay be varied.

1. An ultrasound diagnostic apparatus comprising: an ultrasound probe; atransmission unit for transmitting an ultrasound signal to an object tobe examined via the ultrasound probe; a reception unit for processing asignal received by the ultrasound probe; and an image generating unitfor generating an image on the basis of the received signal processed bythe reception unit, the transmission unit having a function oftransmitting ultrasound signals including a first waveform and a secondwaveform having varying frequency plural times in an identical directionat time intervals, and the reception unit having a function of phasingand adding received signals respectively corresponding to the firstwaveform and the second waveform, wherein the transmission unit has afunction of varying the first waveform so that the frequencytime-sequentially increases and varying the second waveform so that thefrequency time-sequentially decreases and/or the reception unit has afunction of phasing and adding the received signals corresponding to thefirst waveform and the second waveform while amplifying or attenuatingthem with a gain difference.
 2. An ultrasound diagnostic apparatusaccording to claim 1, wherein when the transmission unit has a functionof varying the first waveform so that the frequency time-sequentiallyincreases and varying the second waveform so that the frequencytime-sequentially decreases, the shape of the first waveform and that ofthe second waveform are axisymmetrical about a line perpendicular to atime axis.
 3. An ultrasound diagnostic apparatus according to claim 1,wherein when the transmission unit has a function of varying the firstwaveform so that the frequency time-sequentially increases and varyingthe second waveform so that the frequency time-sequentially decreases,the transmission unit has a function of variably setting a transmissionfocus depth of the ultrasound signal, and a change rate of the varyingfrequencies of the first waveform and the second waveform is variablydetermined depending on the transmission focus depth.
 4. An ultrasounddiagnostic apparatus according to claim 1, wherein when the transmissionunit has a function of varying the first waveform so that the frequencytime-sequentially increases and varying the second waveform so that thefrequency time-sequentially decreases, the first waveform and the secondwaveform vary respectively so that the signal strength is reduced.
 5. Anultrasound diagnostic apparatus according to claim 3, wherein when thetransmission unit has a function of varying the first waveform so thatthe frequency time-sequentially increases and varying the secondwaveform so that the frequency time-sequentially decreases, thetransmission unit has a function of variably setting a transmissionfocus depth, and at least either a change rate of varying frequency ofthe first waveform and the second waveform or a change rate of varyingsignal strength of the first waveform and the second waveform isvariably determined depending on the transmission focus depth.
 6. Anultrasound diagnostic apparatus according to claim 1, wherein when thereception unit has a function of phasing and adding the received signalscorresponding to the first waveform and the second waveform whileamplifying or attenuating them with a gain difference, the gaindifference is variably set depending on a reception timing of thereceived signals.
 7. An ultrasound diagnostic apparatus according toclaim 1, wherein when the transmission unit has a function of varyingthe first waveform so that the frequency time-sequentially increases andvarying the second waveform so that the frequency time-sequentiallydecreases, the reception unit has a filter for extracting apredetermined frequency band of the received signal, and the frequencyband is variably set depending on the reception timing of the receivedsignal.
 8. An ultrasound diagnostic apparatus according to claim 1,wherein when the transmission unit has a function of varying the firstwaveform so as to time-sequentially increase the frequency and varyingthe second waveform so as to time-sequentially decrease the frequency,the frequency of the first waveform and/or the second waveform increasesor decreases at every two or more cycles.
 9. An ultrasound diagnosticapparatus according to claim 1, wherein when the transmission unit has afunction of varying the first waveform so that the frequencytime-sequentially increases and varying the second waveform so that thefrequency time-sequentially decreases, the frequency of the firstwaveform and/or the second waveform increases or decreases at every oneor less cycle.
 10. An ultrasound diagnostic apparatus according to claim1, wherein when the transmission unit has a function of varying thefirst waveform so that the frequency time-sequentially increases andvarying the second waveform so that the frequency time-sequentiallydecreases, the frequency of the first waveform and/or the secondwaveform continuously increases or decreases.